1. Field of the Invention
The present invention relates to a document reader suitable for use in facsimiles, copying machines, information storage systems and others, that is, to an apparatus for converting the image of a document into electric signals.
2. Description of the Prior Art
In the general document readers, a document to be read is irradiated by a lamp to form an image which in turn is projected onto a solid state image sensor such as charge coupled device (CCD) and the like through an imaging optical system. The illumination light from the lamp may be irregular with respect to light distribution on the document. Furthermore, the imaging optical system is placed under the influence of the so-called Cosine.sup.4 Law that as the angle included between the incident light and the optical axis increases, the amount of light passing through the optical system decreases. Due to such various factors, the light distribution in the image formed on the image sensor is generally irregular rather than being uniform. Such an irregularity of the light distribution will be called "shading".
There is known the shading correction in which output signals from the image sensor are corrected in accordance with the shading to form image signals with no influence from the shading. In the conventional shading correction systems, a reflective plate is disposed as a backward plate at the document reading position in which part of the document is to be projected onto the image sensor through the imaging optical system. Before that part of the document reaches this reading position, the reflective plate is irradiated by the light from the lamp to form a reference light reflected by the reflective plate, which reference light is then projected onto the image sensor through the imaging optical system. The resulting output signal from the image sensor is stored in a memory as a reference shading signal. When the document subsequently reaches the document reading position with the image thereof being projected onto the image sensor, the latter generates an electrical signal corresponding to the brightness of the document image formed on the image sensor. This electric signal is then corrected in accordance with the reference shading signal previously stored in the memory. For example, the signal corresponding to the document image is divided by the reference shading signal in a divider circuit. A signal thus obtained from such a correction circuit has no influence from shading.
The shading waveform may be changed to a different one between a point wherein the shading waveform is stored in the memory and another point wherein the signal corresponding to the document image is actually being corrected. If such a difference is too large, a signal obtained from the shading correction will not agree with the part of the document to be read.
To overcome such a problem, the prior art has proposed that a period of time between a point wherein the reference shading signal is stored in the memory and another point wherein a sheet of document has completely been read, that is, wherein the shading correction for a sheet of document has been completed is reduced as little as possible. In transmitters such as facsimiles and others, however, the speed in reading a document highly depends on speeds of its opposite transmitter and line. Thus, a substantial time period may be required to completely read a sheet of document. This is true of the case where a document having an increased length is to be read.
When an increased period of time is required to complete the reading of the document, the aforementioned difference may be increased to form image signals which do not satisfactorily correspond to the document.
When a fluorescent lamp is used as the irradiating lamp and if the ambient temperature is about 0.degree. C., the shading waveform will widely be changed immediately after the system has been started. To overcome this problem, it is required to maintain the fluorescent lamp at its predetermined range of temperature.